1. Field of the Invention
The present invention generally relates to an image scanner and an image forming apparatus, and specifically relates to a sheetfed-type image scanner that employs a contact image sensor to scan a document.
2. Description of the Related Art
A typical contact image sensor includes a contact glass, a white scanning member, a unity-magnification imaging lens, and a scanning sensor. A document is conveyed between the contact glass, and the document is scanned while it passes between the contact glass and the white scanning member. The scanned image is focused on a scanning sensor via a unity-magnification imaging lens arranged near the contact glass.
However, if there is dust or dirt on the contact glass or the white scanning member, a noise occurs in the scanned image. Japanese Patent Application No. 2003-46773 discloses a technique to solve this problem. According to this technique, it is determined, from the scanned image, whether there was dust or dirt on the contact glass or the white scanning member when the document was scanned. If there was dust or dirt on the contact glass or the white scanning member, data is created for correcting the noise in the scanned image due to the presence of the dust or dirt, and the scanned image is corrected by using the created data. The correction of the scanned image arising from the dust or dirt is called the shading correction.
However, noise can occur in the scanned image due to reasons other than the presence of dust or dirt on the contact glass or the white scanning member. For example, noise can occur in the scanned image due to inherent characteristics of the image scanner or due to aging of the image scanner.